Wood is comprised of two main components--a fibrous carbohydrate, i.e., cellulosic portion, and a non-fibrous component. The polymeric chains forming the fibrous cellulose portion of the wood are aligned with one another and form strong associated bonds with adjacent chains. The non-fibrous portion of the wood comprises a three-dimensional polymeric material formed primarily of phenylpropane units, known as lignin. Part of the lignin is between the cellulosic fibers, bonding them into a solid mass, although a substantial portion of the lignin is also distributed within the fibers themselves.
For use in paper-making processes, wood must first be reduced to pulp. Pulp may be defined as wood fibers capable of being slurried or suspended and then deposited upon a screen to form a sheet, i.e., of paper. The methods employed to accomplish the pulping step usually involve either physical or chemical treatment of the wood, or a combination of these two treatments, to alter the wood's chemical form and to impart desired properties to the resultant product. There are thus two main types of pulping techniques, i.e., mechanical pulping and chemical pulping. In mechanical pulping, the wood is physically separated into individual fibers. In chemical pulping, the wood chips are digested with chemical solutions to solubilize a portion of the lignin and thus permit its removal. The commonly utilized chemical pulping processes are broadly classified as: (1) the soda process, (2) the sulfite process, and (3) the Kraft process, with the latter process being most commonly used and being capable of a variety of well-known modifications as described below.
The soda process is well known in the art. It employs sodium hydroxide (NaOH) as the active reagent to break down the lignin and to assist in its removal. The sulfite process is also well known in the art (see, e.g., Handbook for Pulp & Paper Technologists-Chapter 6: Sulfite Pulping (TAPPI, U.S.A.).
The Kraft process together with its numerous variations is the principle chemical process utilized in paper manufacturing. The basic Kraft process, as described in the Handbook For Pulp and Paper Technologists-Chapter 7: Kraft Pulping (TAPPI, U.S.A.), involves digesting the wood chips in an aqueous solution of sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2 S). This process is highly effective in the pulping of even difficult woods such as southern softwoods, as well as the other more readily pulped species of wood such as northern hardwoods and softwoods. The Kraft process likewise generally produces a relatively high-strength pulp since its use results in a diminished attack on the cellulose component of the wood.
The modified Kraft techniques can result in even less degradation in the polymeric structure of the cellulosic fibers during pulping and therefore the strength loss in the resultant paper product is diminished as compared to that occurring with the standard Kraft process. One modified Kraft pulping process is known as "extended delignification", which is a broad term used in the art to encompass a variety of modified Kraft techniques, such as adding the pulping chemicals in a specific defined sequence, or at different locations within the digester apparatus, or at different time periods, or with a removal and reinjection of cooling liquors in a prescribed sequence, so as to more effectively remove a greater amount of lignin while reducing the severity of the pulping liquor's chemical attack on the cellulosic fibers. Another modification of the Kraft process is the Kraft-AQ process, wherein a small amount of anthraquinone is added to the Kraft pulping liquor to accelerate delignification while limiting the attack upon the cellulosic fibers which comprise the wood.
A variety of additional extended delignification techniques are known in the art and include Kamyr Modified Continuous Cooking (MCC) as described by V. A. Kortelainen and E. A. Backlund in TAPPI, vol. 68 (11), 70 (1985); Beloit Rapid Displacement Heating (RDH) as reported by R. S. Grant in TAPPI, vol. 66 (3), 120 (1983); and Sunds Cold Blow Cooking as reported by B. Pettersson and B. Ernerfeldt in Pulp and Paper, vol. 59 (11), 90 (1985).
Digestion of the wood by a Kraft or modified Kraft process results in the formation of a dark colored slurry of cellulose fibers known as "brownstock". The dark color of the brownstock is attributable to the fact that not all of the lignin has been removed during digestion and has been chemically modified in pulping to form chromophoric groups. Thus, in order to lighten the color of the brownstock pulp, i.e., to make it suitable for use as printing and writing and other white paper applications, it is necessary to continue the removal of the remaining lignin by the addition of delignifying materials and by chemically converting any residual lignin into colorless compounds by a process known as "bleaching" or "brightening".
Prior to bleaching the pulp, however, the digested material is conventionally transferred to a separate blow tank after the chemical treatments involved in the pulping process are completed. Within the blow tank, the pressure developed during the initial chemical treatment of the lignocellulosic material is relieved and the pulp material is separated into a fibrous mass. The resulting fibrous mass is then subjected to a series of washing steps to remove the combination of any residual chemicals and the soluble materials (such as the lignin) which were separated from the fibrous materials in the pulping process. Frequently, the pulp also undergoes one or more screening steps designed to separate out the larger portions of undefibered wood for special processing (recooking, mechanical grinding, etc.).
The residue obtained from the washing process, commonly referred to as black liquor, is collected, concentrated, and then incinerated in an environmentally safe manner in a recovery boiler. The technique for the collection, concentration and burning of the black liquor is conventional and is well known in the art.
The delignification and bleaching processes are conducted on the washed fibrous mass in a series of steps, using selected combinations of chemical reactants. In the prior art, various combinations of chemical treatments have been suggested. Furthermore, individual treatment steps have been rearranged in an almost limitless number of combinations and permutations. Therefore, in order to simplify the explanation of the various bleaching processes and systems, the use of letter codes is conventionally employed in combination to describe the particular chemical reactants employed and the sequence of the steps of the process.
The letter codes which will be used hereafter, where appropriate, are as follows:
______________________________________ C = Chlorination Reaction with elemental chlorine in acidic medium. E = Alkaline Dissolution of reaction products Extraction with NaOH. E.sub.o = Oxidative Dissolution of reaction products Alkaline with NaOH and Oxygen. Extraction D = Chlorine Reaction with ClO.sub.2 in acidic Dioxide medium P = Peroxide Reaction with peroxides in alkaline medium. O = Oxygen Reaction with elemental oxygen in alkaline medium. O.sub.m = Modified Uniform alkali treatment of low Oxygen to medium consistency pulp followed by reaction of high consistency pulp with oxygen. Z = Ozone Reaction with ozone. Z.sub.m = Modified Uniform reaction with ozone. Ozone C/D Admixtures of chlorine and chlorine dioxide. H = Hypochlorite Reaction with hypochlorite in an alkaline solution. ______________________________________
O.sub.m and Z.sub.m are modified processes according to the present invention and are described further in the Detailed Description of the Invention.
It has been conventional for many years to delignify and bleach wood pulp by using elemental chlorine. Exemplifying the bleaching of lignocellulosic pulps are the processes disclosed in, for example, U.S. Pat. Nos. 1,957,937 to Campbell et al., 2,975,169 to Cranford et al. and, 3,462,344 to Kindron et al.; and Handbook For Pulp and Paper Technologists-Chapter 11: Bleaching (.sctn.11.3) (TAPPI, U.S.A.).
However, although elemental chlorine has proven to be an effective bleaching agent, it is difficult to handle and potentially hazardous to both mill personnel and equipment. For example, the effluents from chlorine bleaching processes contain large amounts of chlorides produced as the by-product of these processes. These chlorides readily corrode processing equipment, thus requiring use of costly materials in the construction of such mills. Further, the build-up of chlorides within the mill precludes recycling the washer filtrate after a chlorination stage in a closed system operation without employing recovery systems requiring extensive, and therefore expensive, modifications. In addition, concern about the potential environmental effects of chlorinated organics in effluents, which the U.S. Environmental Protection Agency believes to be toxic to humans and animals, has caused significant changes in government requirements and permits for bleach mills which include standards that may be impossible to meet with conventional bleaching or pollution control technology.
To avoid these disadvantages, the paper industry has attempted to reduce or eliminate the use of elemental chlorine and chlorine-containing compounds from multi-stage bleaching processes for lignocellulosic pulps. Complicating these efforts is the requirement that high levels of pulp brightness are required for many of the applications for which such pulp is to be used.
In this connection, efforts have been made to develop a bleaching process in which chlorine-containing agents are replaced, for example, by oxygen for the purpose of bleaching the pulp. The use of oxygen does permit the recycling of effluent from this stage for recovery and does permit a substantial reduction in the amount of elemental chlorine used. A number of processes for bleaching and delignifying pulp with oxygen have been proposed, such as Richter U.S. Pat. No. 1,860,432, Grangaard et al. U.S. Pat. Nos. 2,926,114 and 3,024,158, Gaschke et al. U.S. Pat. No. 3,274,049, Meylan et al. U.S. Pat. No. 3,384,533, Watanabe U.S. Pat. No. 3,251,730, Rerolle et al. U.S. Pat. No. 3,423,282, Farley U.S. Pat. No. 3,661,699, Kooi U.S. Pat. No. 4,619,733 and P. Christensen in "Bleaching of Sulphate Pulps with Hydrogen Peroxide", Norsk Skogindustri, 268-271 (1973). Alkaline pretreatments of pulp prior to oxygen delignification are suggested by U.S. Pat. No. 4,806,203 to Elton.
The use of oxygen, however, is not a completely satisfactory solution to the problems encountered with elemental chlorine. Oxygen is not as selective a delignification agent as elemental chlorine, and the K No. of the pulp, using conventional oxygen delignification methods, can be reduced only a limited amount until there is a disproportionate, i.e., unacceptable, attack on the cellulosic fibers. Also, after oxygen delignification, the remaining lignin has heretofore typically been removed by chlorine bleaching methods to obtain a fully-bleached pulp, but using much reduced amounts of chlorine. However, even at such reduced chlorine concentrations, the corrosive chlorides would soon reach unacceptable concentration levels in a closed cycle operation.
To avoid the use of chlorine bleaching agents, the removal of such remaining lignin with the use of ozone in the bleaching of chemical pulp has previously been attempted. Although ozone may initially appear to be an ideal material for bleaching lignocellulosic materials, the exceptional oxidative properties of ozone and its relative high cost have heretofore limited the development of satisfactory ozone bleaching processes for lignocellulosic materials, especially southern softwoods. Ozon will readily react with lignin to effectively reduce the K No., but it will also, under most conditions, aggressively attack the carbohydrate which comprises the cellulosic fibers and substantially reduce the strength of the resulting pulp. Ozone, likewise, is extremely sensitive to process conditions such as pH with respect to its oxidative and chemical stability, and such changes can significantly alter the reactivity of ozone with respect to the lignocellulosic materials.
Since around the turn of the century, when the delignifying capabilities of ozone were first recognized, there has been substantial and continuous work by numerous persons in the field to develop a commercially suitable method using ozone in the bleaching of lignocellulosic materials. Furthermore, numerous articles and patents have been issued in this area and there have been reports of attempts at conducting ozone bleaching on a non-commercial pilot scale basis. For example, U.S. Pat. No. 2,466,633 to Brabender et al., describes a bleaching process wherein ozone is passed through a pulp having a moisture content (adjusted to an oven dry consistency) of between 25 and 55 per cent and a pH adjusted to the range of 4 to 7.
Other non-chlorine bleach sequences are described by S. Rothenberg, D. Robinson & D. Johnsonbaugh, "Bleaching of Oxygen Pulps with Ozone", Tappi, 182-185 (1975)--Z, ZEZ, ZP and ZP.sub.a (P.sub.a -peroxyacetic acid); and N. Soteland, "Bleaching of Chemical Pulps With Oxygen and Ozone", Pulp and Paper Magazine of Canada; T153-58 (1974)--OZEP, OP and ZP.
Also, U.S. Pat. No. 4,196,043 to Singh discloses a multi-stage bleaching process which also attempts to eliminate the use of chlorine compounds, and includes examples specifically directed to hardwoods. It is well known to those skilled in the art that hardwoods are easier to bleach than most softwoods. This process is characterized by from one to three ozone bleaching stages and a final treatment with alkaline hydrogen peroxide, each stage being separated by an alkaline extraction. One such sequence may be described in the common shorthand nomenclature of the paper industry as ZEZEP. In accordance with this process, the effluent from each treatment stage may be collected and recycled for use in bleaching operations, preferably at an earlier stage than that from which it was obtained. This patent also provides a so-called countercurrent effluent flow.
Despite all of the research conducted in this area, no commercially feasible process for the manufacture of ozone bleached lignocellulosic pulps, especially southern softwood, has heretofore been disclosed, and numerous failures have been reported.
The present invention provides novel combinations of pulping and bleaching steps which overcome the problems encountered in the prior art as discussed herein and which essentially eliminate the discharge of chlorinated organics and minimizes color and BOD releases to produce a high grade bleached pulp in a commercially feasible manner.